1. Field of the Invention
The present invention relates to a mobile communications system, and, more particularly, to a communication system and method for mobile units moving on a road.
2. Description of the Related Art
Recently, studies have been made on various ITSs (Intelligent Transport Systems) to aim at improving the traffic safety and efficiency, traffic environment, etc. To realize such an ITS, it is essential to implement information transmission means, which is the functional core of the system. i.e., an automobile communications system which connects between vehicle-mounted mobile communication devices and roadside transceivers by radio.
Such an automobile communications system may be realized based on the existing mobile telephone system. One example of this automobile communications system is provided with roadside transceivers deployed along a road to segment the road into consecutive radio zones. Such a configuration allows continuous communication with an on-board transceiver of a vehicle running on the road while performing so-called hand-over. Each roadside transceiver is connected to a central station, which gathers information from vehicles and up-links it to a host station or gives information and instructions necessary for road travel to the vehicles. The hand-over command is usually sent from the central station.
A transmission center frequency ft1 and a reception center frequency fr1 are assigned to odd zones in the consecutive radio zones, and a transmission center frequency ft2 and a reception center frequency fr2 are assigned to even zones. The switching of the frequencies is performed to prevent interference between adjoining radio zones. For areas which geographically suffers intense radio wave interference, such as those where radio waves can reach very far, it is necessary to repeatedly assign three or more radio waves.
According to such an automobile communications system, when a vehicle moves from an odd radio zone 1 into an adjoining even radio zone 2, for example, the on-board transceiver should perform so-called hand-over by switching the opposite party from the roadside transceiver of the radio zone 1 to that of the radio zone 2. At this time, the transmission center frequency of the roadside transceiver changes from ft1 to ft2, so that the reception center frequency of the on-board transceiver also changes from ft1 to ft2. This requires that the oscillation frequency of the voltage controlled oscillator of the on-board transceiver should be switched to the center frequency necessary for synchronous detection and demodulation. At the same time, since the reception center frequency of the roadside transceiver changes from fr1 to fr2, the transmission center frequency of the on-board transceiver must also be switched from fr1 to fr2. When the vehicle enters an odd radio zone 3 from the even radio zone 2, likewise, the oscillation frequency of the voltage controlled oscillator of the on-board transceiver should be switched from fr2 to ft1 and the transmission center frequency should be switched from fr2 to fr1.
As apparent from the above, the conventional automobile communications system necessitates switching of the oscillation frequencies of the voltage controlled oscillator of an on-board transceiver and a transmission local oscillator for every hand-over operation of changing the radio zones. In particular, accomplishing fast hand-over requires fast frequency switching, which increases the technical burden on an on-board transceiver. This results in an increased cost for the automobile communication system.